The present invention relates to a process for manufacturing a semiconductor device having a crystalline semiconductor. The present invention further relates to thin film transistors (TFT) used in integrated circuits, especially, for switching elements in an active matrix circuit in an electro-optical device or a driving circuit thereof formed on the same substrate as the active matrix circuit.
An amorphous silicon film can be utilized most readily as the thin film semiconductor for a TFT. However, the electric characteristics of the amorphous silicon film are poorer than a crystalline thin film semiconductor, for example, polycrystalline silicon, single crystalline silicon, and microcrystalline silicon. A crystalline silicon film can be prepared by first forming an amorphous silicon film, and then heat treating the resulting film for crystallization.
The heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600.degree. C. or higher for a duration of 10 hours or longer. Such a heat treatment is detrimental for a glass substrate. For instance, a Corning 7059 glass commonly used for the substrate of active matrix liquid crystal display devices has a glass distortion point of 593.degree. C., and is therefore not suitable for large area substrates that are subjected to heating at a temperature of 600.degree. C. or higher.
According to the study of the present inventors, it was found that the crystallization of an amorphous silicon film can be effected by heating the film at 550.degree. C. for a duration of about 4 hours by the provision of a trace amount of nickel or palladium, or other elements such as lead, onto the surface of the amorphous silicon film.
The elements above (hereinafter referred to as "catalyst elements capable of promoting the crystallization of an amorphous silicon film" or simply as "catalyst elements") can be introduced into the surface of the amorphous silicon film by depositing the elements by plasma treatment or vapor deposition, or by introducing the elements by ion implantation. The plasma treatment is a treatment in which the catalyst elements are added into the amorphous silicon film by generating a plasma in an atmosphere such as gaseous hydrogen or nitrogen using an electrode containing the catalyst elements therein in a plasma CVD apparatus of a parallel plate type or positive columnar type.
However, the presence of the catalyst elements in a large quantity in the semiconductor is not preferred, because the use of such semiconductors greatly impairs the reliability and the electric stability of the device in which the semiconductor is used.
That is, the catalyst elements are necessary in the crystallization of the amorphous silicon film, but are preferably not incorporated in the crystallized silicon so much. In order to satisfy these requirements, it is necessary to select an element which tends to be inactive in crystalline silicon as the catalyst element, and to make the amount of the element added in the silicon film minimum. For this purpose, the quantity of the catalyst element to be incorporated in the film must be controlled with high precision.
The crystallization process using nickel was studied in detail. The following findings were obtained as a result:
(1) In case of incorporating nickel by plasma treatment into an amorphous silicon film, nickel is found to intrude into the film to a considerable depth of the amorphous silicon film before subjecting the film to a heat treatment;
(2) The initial nucleation occurs from the surface at which nickel is added; and
(3) When a nickel layer is formed on the amorphous silicon film by vapor deposition, the crystallization of an amorphous silicon film occurs in the same manner as in the case of effecting a plasma treatment.
(4) When incorporating a relatively large amount of nickel into an amorphous silicon film, if a laser is irradiated onto the amorphous silicon film for crystallization or after a heat crystallization thereof, the nickel tends to segregate at the surface of the film, resulting in that such a film can not be used as an active semiconductor layer.
In view of the foregoing, it is assumed that not all of the nickel introduced by the plasma treatment functions to promote the crystallization of silicon. That is, if a large amount of nickel is introduced, there exists an excess amount of the nickel which does not function for promoting the crystallization. For this reason, the inventors consider that it is a point or a face at which the nickel contacts the silicon that functions to promote the crystallization of the silicon at lower temperatures. Namely, it is assumed that the nickel has to be minutely dispersed in the silicon in the form of atoms. Namely, it is assumed that nickel needs to be dispersed in the vicinity of a surface of an amorphous silicon film in the form of atoms, and the concentration of the nickel should be as small as possible but within a range which is sufficiently high to promote the lower temperature crystallization.
A trace amount of nickel, i.e., a catalyst element capable of accelerating the crystallization of silicon can be incorporated in the vicinity of a surface of an amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous concerning the controllability of the film, and is therefore that is not suitable for precisely controlling the amount of the catalyst element to be incorporated in the amorphous silicon film.